Currently, much attention is focussed on communication systems such as A (Asymmetric) DSL, S (Symmetric) DSL, V (Very high speed) DSL, H (High speed) DSL, and the like, as methods for carrying out data communication by using telephone lines. Accompanying this trend, research and development are being carried out on carrying out data communication simultaneously at the same line, while using even conventional telephones. When data is multiplexed with a telephone, in order for conventional telephone service to not be adversely affected, it is necessary to carry out design for the xDSL modem while paying very close attention. It is desirable to reliably carry out telephone service even if some functions must be sacrificed.
On the other hand, in a case in which telephone service is not needed, it is desirable to carry out communication faster. In this case, when using an xDSL modem which is the same as in the case when multiplexed with telephone service, useless frequency space arises, which is not efficient. The reasons for this are described hereinafter.
Namely, when telephone service and data communication service are multiplexed on the same line, the line capacity of the modem is specified by recommendation, For example, in ITU-T Recommended G.992.1 Annex E, the line capacity is specified to be 35 nF, and the cut-off frequency is 45 KHz. In contrast, with data communication which is not multiplexed with telephone service, there is no such stipulation. However, when a cut-off frequency of 45 KHz is applied as is to data communication which is not multiplexed with telephone service, the band in a region lower than 45 KRz cannot be utilized, which is inefficient.
FIG. 4 is a structural diagram of a case in which telephone service and data communication service are multiplexed on the same telephone line. FIG. 5 is a structural view of a case in which communication service is not multiplexed with telephone service. In FIG. 4, a data terminal 16 is connected to a telephone line 13 via an xDSL modem 15 and a branching filter 14. Further, a telephone 19 is also connected to the telephone line 13 via the branching filter 14.
The telephone line 13 is connected to a data network 10 via a branching filter 12 and an xDSL modem 11 in a station of a communication provider. Further, the telephone line 13 is connected to a public network 17 via the branching filter 12 and an exchanger 18. In the case illustrated in FIG. 4 in which data communication service and telephone service are multiplexed, the line capacities of the modems 11, 15 are 35 nF, in accordance with the previously-mentioned ITU-T recommendation. Therefore, the cut-off frequency for communication data is 45 KHz.
On the other hand, in the case shown in FIG. 5 in which data communication service is not multiplexed with telephone service, a data terminal 24 is connected to a telephone line 22 via an xDSL modem 23, and the telephone line 22 is connected to a data network 20 via an xDSL modem 21 at a station of the communication provider. Also in FIG. 5, in a case in which the aforementioned specified 35 nF is used as the line capacities of the modems 21, 23, the cut-off frequency must be 45 KHz. Therefore, regardless of the tact that data communication service is not multiplexed with telephone service, the use of the band in a region lower than 45 kHz is limited, which is inefficient.
Therefore, conventionally, a technique has been used in which two different types of modems are used, i.e., the modem of the type used for the system of FIG. 4 and the modem used for the system of FIG. 5, and the appropriate modem is used in accordance with the system, which is also inefficient.